It is well known that polyurethanes can be used as binders for high quality coating compositions. The resulting coatings have excellent mechanical properties, particularly with regard to the resistance of the cured paint coat to mechanical stress.
Consequently, polyurethanes are frequently used in the manufacture of conventional motor vehicle paint coats.
Such a motor vehicle paint coat generally consists of a total of four different layers (quadruple layer structure). These four layers are applied one after the other in separate coating facilities.
The first layer which is present directly on the motor vehicle sheet metal consists of a layer applied by electrophoresis (electrocoat layer, cathodic electrodeposition layer) which is applied by electrodeposition coating—mainly cathodic dip coating (CDC)—for protection against corrosion and subsequently stoved.
The second coat, approximately 30 to 40 μm thick, which is present on top of the electrocoat layer, is a so-called filler layer which, on the one hand, provides protection against mechanical attack (protection against chipping) and, on the other hand, guarantees satisfactory top coat non-sag properties, i.e. it smoothes the rough surface of the blank motor vehicle body for subsequent top coating and fills small unevennesses. The paints used to produce this filler layer contain not only binders but also pigments. In this connection, the wettability of the pigments used influences the top coat non-sag properties of the multi-layer coating as a whole and also the gloss of the filler layer such as it is required by some motor vehicle manufacturers. The filler layer is largely produced by application by means of electrostatic high rotation bells and a subsequent stoving procedure at temperatures above 130° C.
The third layer present on the filler layer is the base coat layer which provides the desired colour for the motor vehicle body by means of corresponding pigments. The base coat is applied by the conventional spray method. The layer thickness of this conventional base coat layer is between approximately 12 and 25 μm, depending on the tint. Usually, this layer is applied in two process steps, particularly in the case of metallic paints. In a first step, the application takes place by means of electrostatic high rotation bells, followed by a second application by pneumatic atomisation. This layer is subjected to intermediate drying by infrared radiators and/or hot air convection (when an aqueous base coat is used). The fourth and uppermost layer which is present on the base coat layer is the clear coating layer which is usually applied by electrostatic high rotation bells in one application. It provides the motor vehicle body with the desired gloss and protects the base coat against environmental effects (UV radiator, salt water etc.)
Subsequently, the base coat layer and the clear coating layer are stoved together.
The fillers used for the manufacture of a multi-layer coating for the motor vehicle industry are still based to a large extent on solvents and reach a solids concentration of up to 60%. This high solids concentration guarantees an efficient application and consequently satisfactory top coat non-sag properties of the finished multi-layer coating. Examples of such conventional fillers are known from DE 33 37 394 A1.
As a rule, the stoving temperatures are between 155 and 165° C. Some motor vehicle manufacturers demand an additional so-called “overstoving stability” up to 190° C. This means that the mechanical properties, such as adhesion and resistance to chipping, must not be subject to major deterioration under these demanding stoving conditions.
In order to satisfy these requirements, the binder compositions of corresponding filler systems frequently consist of saturated polyesters in combination with highly alkylated melamine resins as crosslinking agents. Combinations with polyurethanes, in particular with blocked polyisocyanates, are also known. The blocking agent most frequently used for this purpose is methyl ethyl ketoxim. The advantage of using methyl ethyl ketoxim compared with other blocking agents consists of its favourable deblocking temperature, its volatility and its satisfactory availability. A disadvantage is its tendency towards yellowing which greatly restricts the suitability for use for light-coloured stoving enamels.
Although crosslinking with blocked polyisocyanates improves the resistance to chipping and the overstoving stability, the simultaneous use of melamine resins as crosslinking agents is essential in view of other important properties of the filler layer such as high suitability for rubbing down, resistance to chemical abrasion, e.g. vis-à-vis brake fluid. Moreover, the levelling properties and top coat non-sag properties are influenced positively by a combination of these two crosslinking agents.
However, such hybrid systems of blocked polyisocyanates and melamine resins cause problems with respect to the correct adjustment of their reactivity: whereas the deblocking reaction in the case of polyurethanes and the subsequent urethane formation reaction are promoted by basic catalysts, the crosslinking reaction of melamine resins can be accelerated only with acidic catalysts. Reciprocal negative influences are unavoidable.
Against the background of the solvents used in conventional fillers and the environmental problems connected therewith, increased developments can be observed in the field of water-thinnable fillers based on polyurethanes.
In order to make polyurethanes dispersible in water, carboxyl groups are incorporated into the molecule which—frequently at the end of the synthesis process—are converted into carboxylate anions by neutralisations with (preferably volatile) amines. Very frequently this incorporation of carboxyl groups is also effected by conversion with dimethylol propionic acid.
Examples of such water-thinnable fillers containing corresponding polyurethanes are known from EP 0 726 919 A1, EP 0 594 685 B1, EP 1 110 983 A2 und EP 1 110 987 A1.
In addition, polyurethanes with blocked isocyanate groups are disclosed in DE 199 30 555 C1, which polyurethanes are obtainable by using alkanolamines as chain extenders. In the case of this chain extension, the hydroxyl groups of the alkanolamine react with the NCO groups of the polyisocyanate forming a urethane bond.